You asked for advice, but it doesn't seem like you like the answers.
. That's OK, of course.
If you want to be a landlord, I'd read a few books on the ins and outs. We looked very closely at the idea a few years ago and decided not to. The amount of work was too high, and the expected return on our dollars was too low, given the risks. Getting a mortgage is a tiny issue compared to your need to get educated on all the risks, tax implications, and so on. It's not simple, in my opinion. Quick forum answers won't prepare you.
I agree with others that you shouldn't count on your son being part of this equation. What if he wants to move? There are no guarantees. He is no more than one possible tenant in this scenario.
Good luck to you. If you do decide to move ahead, check out your local rental housing association for classes.